The effect of Ti[O.sub.2], pigmentation on the hydrolysis of amino resin crosslinked epoxy can coatings.
Keywords Ti[O.sub.2], Melamine, Coatings, Amino, Crosslinker, Release, Hydrolysis
Various pigmented coatings are formulated and used in the can coating industry. Pigmentation usually affects the mechanical strength of the coatings. With internal can coating lacquers, after application, the coating lies between the metal and the contents of the container, providing protection for packaged contents from spoilage and/or microbial attack. (1,2) Such internal coatings are required to be coherent, continuous, and free from pin holes. Good flexibility in the coating and good adhesion between the coating and the metal substrate are vital requirements. The coatings need to be fit for purpose, for a required, significant time. (1)
Ti[O.sub.2] is the most widely used white pigment in the can coating industry. (3,4) Ti[O.sub.2] pigments are used in the coatings to enhance/modify many properties of the coatings including corrosion resistance, flow characteristics, appearance, thermal properties, and mechanical properties. Also, Ti[O.sub.2] is used to help promote interfacial interactions between some of the components of the coatings, allowing the coatings to perform their required functions effectively. (5) The properties of Ti[O.sub.2] include its high opacity and high refractive index. Ti[O.sub.2] is produced in various particle sizes. This particle size and the size distribution govern the various applications to which the white pigment can be subjected. Ti[O.sub.2] is produced in two major crystal forms, termed the anatase and the rutile. The latter possesses a greater opacity. Both grades have coating-based applications. (7)
Different qualities and forms of Ti[O.sub.2] exist for commercial application. These are related to the required loading of the pigment, the particle size ranges, the durability, the presence of additives and so on, designed to meet particular performance needs. The application to which a Ti[O.sub.2]-pigmented coating might be subjected determines the grade choice. (4) Ti[O.sub.2] pigments, when used in can coatings, are expected to provide effective chemical stability, barrier provision, durability, good dispersion quality/stability, and appropriate esthetics (appearance). (8,9) Most importantly, in pigmented coatings for cans, the grades of Ti[O.sub.2] must be suitable for food contact application across the production, storage, and use cycles.
During the processing of many canned food items, thermal treatments of the can and contents are applied. These include sterilization (at temperatures up to 121[degrees]C for few seconds) and pasteurization (at temperatures up to 80[degrees]C for several, usually, specified minutes, depending on food type). (10) As a result, the possibilities arise that physical changes and chemical changes could take place in the coating, resulting in unwanted interaction/reaction (physical and/or chemical) between the metal substrate, the internal coating, and the food contents, leading to the potential generation/release of unwanted by-products that could give rise to health issues and food quality issues. (11)
To ensure that fitness for purpose criteria are met in the use of some widely used Ti[O.sub.2] pigments in the can coating industries, a study of the effect of the presence of the Ti[O.sub.2] pigments when incorporated into epoxy anhydride coatings on the hydrolysis of the retorted coatings was undertaken. Pigmented coatings and unpigmented coatings were formulated and studied. In addition, investigations of the consequences of incorporating or excluding Ti[O.sub.2] pigment particles from amino resin-based can coatings on the release of melamine into aqueous food simulants were investigated. In addition, effects of aging of the coatings were monitored. Also the effects of the different extents of surface treatment of the Ti[O.sub.2] pigment particles were studied.
The tinplate substrates (TH415) were manufactured by Arcelor Mittal and have a thickness of 0.21 mm. The coating components, namely the three food grade Ti[O.sub.2] pigments, the polymeric binders, a flow additive, the specified amino crosslinkers and the Naptha light aromatic solvent composition, were supplied by Valspar Inc. Surfynol CT-324, as a surfactant was obtained from Air Products Ltd. Acetonitrile (HPLC grade, 99.9%), dimethyl sulphoxide (99%), melamine (99%), sulphuric acid (99.9%), chromotropic acid disodium salt dihydrate (99%), methanol (99%), propylene glycol monomethyl ether acetate (96%), 2-n-butoxyethyl acetate (98%), acetic acid (glacial), and ethanol (99%) were purchased from Sigma Aldrich, UK. Cyclohexanone (99%) and sodium dihydrogen phosphate monohydrate (99%) were purchased from Alfa Aeser, UK. 2-butoxyethanol (98%) was purchased from Acros Organic, UK. All of the chemicals were used as received. Doubly distilled, deionized water, discharged from an Elga, 18 [OMEGA], Purelab-flex-Velolia distiller, was used in the preparation of the aqueous, ethanolic food simulants.
Analysis of Ti[O.sub.2] pigments
For the particle size analysis of the Ti[O.sub.2] pigments, a Malvern Instruments Zetasizer (Nano-ZS) was used. In each case, approximately 1 g of pigment was transferred into a beaker. 1 mL of Surfynol CT-324 surfactant was added to act as a dispersant/wetting agent, before mixing, until a paste was formed. 40 mL of doubly distilled, deionized water was then added into the beaker. The mixture was stirred and ultrasonicated for 5 min, after which time a suspension was formed. Fifteen drops of the formed suspension were then added into a beaker containing 25 mL of doubly distilled water. This mixture was then shaken and stirred. Portions were then transferred into plastic cuvettes for the particle size measurements. Portions of the mixture were also used for the measurement of the zeta potential of the pigment dispersions, using a Malvern Instruments Zetasizer (Nano-ZS).
The hydrophilic nature of each of the three food grade Ti[O.sub.2] pigment particles was determined because of the need to know the extent to which the different grades of the pigment retained moisture. 1 g of each pigment grade was mixed with 1 mL of distilled, deionized water until a pigment paste was formed. Each paste was applied onto a watch glass and allowed to "dry" under a forced draught, at 20[degrees]C, for 24 h. Portions of the samples were taken and evaluated using thermogravimetric analysis (TGA). The thermograms were recorded using a TA Universal Instrument V4.1D, over a temperature range of 0-500[degrees]C, and a heating rate of 10[degrees]C/min. Approximately 10 mg of each sample was used for each evaluation.
In order to observe the morphology in each of the Ti[O.sub.2] pigment grades, a JOEL JSM-6610LV, Oxford Instruments INCA-X-Max 80 EDS unit was used. For the SEM analyses, small portions of each pigment were used. Each sample was mounted on a brass stub and then surface pretreated to a uniform 30-nm gold film deposition, using a Bio-Rad diode sputter coating unit, (Bio-Rad House, Hertfordshire, UK), before being evaluated by SEM.
The metal atom contents and the metal atom ratio in the pigment samples were evaluated using the EDX components of the JOEL JSM-6610LV, Oxford SEM Instruments. 100 frames of data were collected for each sample using an accelerating voltage of 15 kV, under the aperture 3 mode of operation. In each case, a selective elemental distribution for a defined microscopic area was monitored.
Preparation of the "model" epoxy-anhydride coatings
Dried, solid BPA-based epoxy resin binder (40.0 g) and the dried anhydride-based polymer (4.4 g) were mixed with propylene glycol monomethyl ether acetate (23.6 g), 2-n-butoxyethyl acetate (30.0 g), and cyclohexanone (2.0 g), using a Heidolph mixer (Type R), to form the polymer solution. To a 55.5 g of the polymer solution, under stirring, was added the dispersant (0.25 g), the specified Ti[O.sub.2] pigment (23.5 g), 2-butoxyethyl acetate (4.0 g), 2-butoxyethanol (3.85 g), one option of the amino crosslinkers (0.5 g, Table 2), propylene glycol monomethyl ether acetate (0.2 g), flow additive (0.2 g) and the naphtha light aromatic solvent (1.5 g). For comparison purposes, the procedure was repeated without incorporating any Ti[O.sub.2] pigment. In Figs. 7 and 8, the epoxy anhydride coatings containing the pigment are denoted as PEA coatings, while those without pigment are coded UEA coatings. Some of the characteristics of the three Ti[O.sub.2] pigments used in formulating the coatings are given in Table 1. Details relating to the amino crosslinkers that were used in the formulation series are shown in Table 2. Crosslinkers of the same class exhibit different values of viscosity because of differences in the extents of reaction and levels of oligomerization during their synthesis.
Coating application and curing
A4-sized sheets of a tinplated substrate were used. The Ti[O.sub.2] pigmented, epoxy-anhydride, coatings were applied to the substrates using a K-bar coater, No. 3. The unpigmented epoxy-anhydride coatings were applied to tinplated substrates using a K-bar coater, No. 4. The application of the pigmented coatings and the unpigmented coatings was designed to provide similar amounts of organic material in the fluid coatings for curing. The coated metal tinplates were loaded on to the carrier drive of a Werner Mathis curing oven (model--KTF4099) and driven into the oven at a speed of 2 m/min. After the appropriate cure time had passed, the panels were removed, allowed to cool, and made ready for evaluation.
Surface microscopy of coatings and contact angle studies: epoxy-anhydride coatings
Microscopic observation of the surface of the three FIMMM-based, crosslinked cured epoxy-anhydride coatings, each containing one of the three Ti[O.sub.2] pigments, was carried out using a JOEL JSM6610LV, Oxford Instruments INCA-X-Max 80 EDS. Small pieces of the coating panels were carefully cut to allow them to be mounted onto the brass stub of the Bio-Rad diode sputter coating unit, (Bio-Rad House, Hertfordshire, UK). These were then pretreated to a uniform 30-nm gold film deposition, using the coater, before being evaluated by SEM.
With respect to the food-can coating contact issues, in order to provide a basis for the study of interactions at liquid-to-solid interfaces, contact angle studies were carried out using 10% (v/v) aqueous ethanol and 3% (v/v) aqueous acetic acid as food simulants. These are the food simulants that were used in the sterilization, autoclaving studies. A small drop (sessile drop) of each of the simulants was applied to the cured coating panels and the static contact angle that the fluid made on the panel surface was recorded using a Livereel-contact-[theta]-Meter. Each measurement was repeated three times and an average value was adopted in each case, on the basis that each value was within the acceptable accuracy limits.
Retort of coatings/hydrolysis tests: epoxyanhydride coatings
In a typical procedure, a 5 cm x 5 cm area of coated, cured tinplate was immersed in 10 mL of 10% (v/v) of aqueous ethanol, (as a food simulant), in a glass vial. The container was then sealed using a crimper and placed into a pressure vessel. The pressure vessel was then heated to 131 [degrees]C and maintained at this temperature for 60 min. In the same manner, retorts were also carried out using 3% (v/v) aqueous acetic acid as food simulant. After the exposure time, the pressure vessel was cooled to room temperature. Then, an aliquot of the simulant was taken for the analysis of its melamine content.
The concentration ([micro]g/mL) of the melamine migrant was detected from each extraction solution and then converted into migration values ([micro]g/6 [dm.sup.2]), taking into consideration the exposed area (A) of coatings ([dm.sup.2]) and the volume (E) of aqueous food simulant (mL). The extent of crosslinker hydrolysis to melamine was calculated according to equation (1).
% Hydrolysis of crosslinker = (EY/TY) x 100% (1)
In equation (1), EY refers to the experimental migrant yield in [micro]g/6 [dm.sup.2]. TY refers to the theoretical migrant yield ([micro]g/6 [dm.sup.2]) taking into consideration the maximum amount of the melamine that could result from the complete hydrolysis of the amino crosslinker that was used in any particular coating formulation. TY of melamine was calculated according to equation (2)
TY = MDV x SMR x FWT x %CRL (2)
In equation (2), MDV is the melamine degradation value, representing the conversion of the crosslinker into melamine. Thus, as an example, one mole of hexamethoxymethyl melamine (HMMM: 390 g/mol) will give rise to 1 mol of melamine (126 g/mol). SMR is the conventional surface area to mass ratio of 6 [dm.sup.2]. FWT is film weight thickness (g/[m.sup.2]). The %CRL is the wt% of the crosslinker in the total wet coating formulation.
Determination of the melamine released by the epoxy-anhydride coatings
For the determination of the melamine content in the original crosslinkers and of that released by the coatings, the HPLC technique was used. The column used was a Hypersil C18, N 5 [micro]m, 2.1 x 300 mm. The column temperature was 25[degrees]C. The injection volume was 10 [micro]L and the detection was by UV spectroscopy. Two eluents were used, based on a combination of 100% acetonitrile and a 5 mM phosphate buffer, pH 6.5. A flow rate of 1 mL/min (using 5% acetonitrile and 95% of the phosphate buffer) was used for the 10 min run time. In the analysis of the melamine in the crosslinkers, 1 g of each crosslinker sample was weighed and dissolved in 50 mL of methanol, in a volumetric flask. Portions were then taken for analysis by HPLC, to determine the amount of any melamine that might have been present.
Results and discussion
Behavior of the Ti[O.sub.2] pigment species
The characteristic properties of the three grades of the Ti[O.sub.2] pigments were monitored. Thus, SEM images, TGA profiles, elemental distributions, particle size values, and zeta potential values were obtained and compared.
The SEM images obtained for the three Ti[O.sub.2] pigment grades used, Ti[O.sub.2] 1, Ti[O.sub.2] 2, and Ti[O.sub.2] 3 are shown in Fig. 1. The shapes of the primary particles for each pigment grade appear to be roughly spherical and reasonably evenly distributed and uniform. However, Ti[O.sub.2] 1 and Ti[O.sub.2] 3 show some aggregated clusters of particles.
The water retention behavior, providing a guide to the potential hydrophilicity of the three pigment grades, was compared using TGA, under three different heating temperatures, as shown in Figs. 2, 3, and 4. In each figure, the weight loss is shown as a function of the heating time, at the stated heating rate. The pigment types show moisture retention, even when their pastes had been "dried" at room temperature for 24 h. Thus, they can be described as having a limited degree of hydrophilic behavior. According to Figs. 2, 3, and 4, the behavior of the three pigments is very similar.
In Table 3, the Ti (%) and the Al (%) content of each of the three Ti[O.sub.2] pigment grades is presented. The results relating to the % of Ti should be considered to be semiquantitative because of instrument sensitivity issues. It is clear from the results that the different grades of the pigment contain different amounts of aluminum atoms. According to Fig. 5, the relative amount of water lost by the pigment pastes after heating isothermally for 20 min was greatest at 300[degrees]C. For heating at this temperature also, the amount of water lost by the pigment pastes linearly increases with the amount of aluminum in the pigments. Overall, the points that are noteworthy from Fig. 5 are that the pigments have a significant strength of binding to the hydrophilic agent (water). All the pigments show their water retention behavior and can therefore be considered to have limited hydrophilicity. The greatest water retention was observed for the pigment pastes with the highest content of alumina (2.18% aluminum), studied at the three different temperatures.
The particle size distribution, zeta potential distribution, IR spectra, and SEM images of the three Ti[O.sub.2] pigment grades are all very similar (Figs. 6, 7, 8, and 9). Surface-treated-Ti[O.sub.2] particles can have zeta potential values in the range of -54 to -72.7 mV, depending on the depth/type of the treatment. (12) Figure 8 shows that the three pigment grades did not significantly influence the chemical composition of the different coatings. The average particle sizes obtained for Ti[O.sub.2] 1, Ti[O.sub.2] 2, and Ti[O.sub.2] 3 were 318, 310, and 355 nm, respectively. The average size distribution of the three pigments was above 100 nm, the value categorized for nanomaterials. There is still controversy and debate concerning the consequence of the migration of nanoparticles from packing into food. (13) The Ti[O.sub.2] pigments used in this study, therefore, avoid such controversy.
The SEM micrographs of the surface of the cured, HMMM crosslinked epoxy anhydride coatings, Fig. 9, show that, in each case, the coatings were applied and cured uniformly. They also show that the pigment grades were well dispersed in the coatings and that uniform coating layers were achieved. In each case, at the magnification that was used, there is no evidence of voids, surface irregularities, surface porosity, or any major surface defects.
Effect of pigmentary Ti[O.sub.2] on the hydrolysis of cured epoxy-anhydride coatings to melamine
The effect of the use of the different Ti[O.sub.2] pigment-grades on the amounts of melamine that were generated and released during the retorting of epoxy-anhydride coatings on cans was investigated. Also, the effects of aging the fluid coating on melamine release and crosslinker hydrolysis were compared. Comparison of the results from the crosslinker hydrolysis studies show that the amounts of melamine that the coatings released after retorting can be put into context with respect to the European acceptable limits for food contact materials.
Figures 10 and 11 show the release of melamine from the epoxy-anhydride coatings that occurred when either of the food simulants was used, i.e., the 3% (v/v) aqueous acetic acid or the 10% (v/v) aqueous ethanol. There was a slightly greater release of melamine when the 3% (v/v) aqueous acetic acid food simulant was used, because of its greater acidity, although both food simulants are protic solvents. (14)
For can coatings, a 6 months shelf life is typically set as one criterion for fitness of purpose. In this study, storage times of 20 and 40 weeks were used to represent the time that elapses before the coatings are used in can coating processes (Valspar Europe). Table 4 shows the results that were obtained from investigations of the aging behavior of fluid, pigmented epoxy anhydride coatings. The influence of the controlled aging of the coatings on melamine release and on crosslinker hydrolysis was monitored.
The results in Table 4 show that each of the pigmented coatings underwent aging, as observed by studying the melamine release from the epoxy anhydride coatings. The wet aging effect is significant, as shown by the fact that quality control measures were taken during the sample storage and during the investigations. Also, the tests were done 40 weeks apart. In each case, both the release of melamine and the hydrolysis of the crosslinker increased as a consequence of the aging. The results show that, during storage, the pigmented epoxy anhydride coatings, each containing a different pigment grade, behaved very similarly. Also, the results presented in Figs. 12 and 13 show that at 40 weeks, the liquid coating exhibited a slight increase in thermal stability, as a result of time-dependent transformations, during storage. It is possible that at the storage temperature slow structure development commences during the storage period and was most pronounced after 40 weeks. The coatings that were aged up to 40 weeks were more viscous than the others. Due to the self-crosslinking tendency of the melamine crosslinkers, (15) any such reaction that might occur during the aging process will reduce the subsequent curing of the aged coating and this will alter the overall extent of crosslinker hydrolysis during the retorting of the cured coatings in the food simulants.
The results in Table 4 indicate that the three pigment grades can be used interchangeably with respect to the melamine release behavior that arises during the hydrolysis of the cured coatings. Even with aging, the melamine released by the coatings was well below the current European acceptable limit of 2.5 mg/6 [dm.sup.2]. (16) However, it is expected that the limit will be reduced in line with Organisation for Economic Co-operation and Development (OECD) recommendations. Since any release of melamine is unwanted, the significance of the age of components in the coatings, the mixing quality and the pigmentation could offer guidelines for good formulation practice. Such aging should become a component of all "fitness for purpose" related testing.
According to the results given in Table 5, when the Ti[O.sub.2] particles were excluded from the coatings, the amount of melamine that was released through hydrolysis increased significantly. This observation was consistent across all the coatings, each of which contained a different grade of amino crosslinker. Thus, the polar, hydrophilic surface of the Ti[O.sub.2] particles could influence the polymerization process that occurs between the melamine crosslinkers and the epoxy-anhydride polymeric binders that exist within the coating. In this way, a greater crosslink density could be achieved, resulting in the lesser melamine release observed. The Ti[O.sub.2]-pigmented epoxy anhydride coatings gave a lower overall release of melamine at each stage of the analysis than was released by unpigmented epoxy anhydride coatings. According to Hosseinpour et al., (17,18) the surface treatment of titanium dioxide can have a profound influence on pigment--polymer interfacial interactions, on stability factors, and the mechanical properties of coatings.
The three Ti[O.sub.2] pigment grades that were characterized and used in epoxy-anhydride can coating formulations influenced the melamine release behavior and cross-linker hydrolysis of the amino coatings to the same extent, both with fresh formulations and with aged formulations. The three pigment grades can be effectively used interchangeably in epoxy-anhydride coatings with respect to any melamine release behavior. Aging of liquid coatings increased the amount of melamine that the coatings released during the retorting of the cured coatings in the aqueous food simulants. The overall hydrolysis behavior of the epoxy coatings depends on the chemistry of the amino crosslinker that is used in the formulations. The incorporation of Ti[O.sub.2] pigment particles into epoxy coatings offers a potential for reducing the extent of melamine that is released both during after coating retorting/sterilization. Thus, effective formulation strategies with respect to the use of Ti[O.sub.2] pigments and the choice of amino crosslinker type can provide partial control on the hydrolysis of selected epoxy coatings.
(1.) Magami, SM, Guthrie, JT, "Amino Resin Cross-linked Can Coatings." Surf. Coat. Int., 95 (2) 64-73 (2012)
(2.) Oldring, PKT, Nehring, U, Packaging Materials 7. Metal Packaging for Foodstuffs. ILSI Europe, Belgium (2007)
(3.) Arellano, M, Michel-Hacisi, I, Feke, DL, Monas-Zloczower, IM, "Compatibilization of Titanium Dioxide Powders with Non-polar Media: Adsorption of Anionic Surfactants and Its Influence on Dispersion Stability." J. Coat. Technol., 68 (857) 103-112 (1996)
(4.) Braun, JH, "Titanium Dioxide--A Review." J. Coat. Technol., 69 (868) 59-72 (1997)
(5.) Al-Turaif, HA, "Effect of Nano Ti[O.sub.2] Particle Size on Mechanical Properties of Cured Epoxy Resin." Prog. Org. Coat., 69 (3) 241-246 (2010)
(6.) Rout, MTK, Sengupta, DK, "Electrokinetics of Ti[O.sub.2] in the Interpretation of Its Dispersion Characteristics." Surf. Coat. Int., 83 (12) 592-596 (2000)
(7.) Reck, E, Richards, M, "Titanium Dioxide-Manufacture, Environment and Life Cycle Analysis, the Trioxide Experience." Surf. Coat. Int., 80 (12) 568-572 (1997)
(8.) Ahmad, S, Ashraf, SM, Kumar, GS, Hasnat, A, Sharmin, E, "Studies on Epoxy-Butylated Melamine Formaldehyde-Based Anticorrosive Coatings from a Sustainable Resource." Prog. Org. Coat., 56 (2-3) 207-213 (2006)
(9.) Schaefer, A, Mass, S, Simat, TJ, Steinhart, H, "Migration from Can Coatings: Part 1. A Size-Exclusion Chromatographic Method for the Simultaneous Determination of Overall Migration and Migrating Substances Below 1000 Da." Food Addit. Contam., 21 (3) 287-301 (2004)
(10.) Magami, SM, "The Science and Technology of Can Coatings." Surf. Coat. Int., 95 (3) 139-142 (2012)
(11.) Bradley, EL, Castle, L, Day, J, Leak, J, "Migration of Melamine from Can Coatings Cross-Linked with Melamine-Based Resins, into Food Simulants and Foods." Food Addit. Contam., 28 (2) 243-250 (2011)
(12.) Sheen, S, Yang, S, Jun, K, Choi, M, "One-Step Flame Method for the Synthesis of Coated Composite Nanoparticles." J. Nanopart. Res., 11 (7) 1767-1775 (2009)
(13.) European Commission, Institute for Health and Consumer Protection, "Nanomaterials in Food Packaging: Keeping Food Fresh for Longer, Less Waste." http://ihcp.jrc.ec.europa. eu/our_activities/nanotechnology/Nano_Food/nanomaterials-in-food/?searchterm=None. Accessed August 05, 2013
(14.) Brown, WH, Organic Chemistry, 7th ed. Cengage Learning Publishers, London (2013)
(15.) Jonesa, FN, Chua, G, Samaraweera, U, "Recent Studies of Self-condensation and Co-condensation of Melamine-Formaldehyde Resins; Cure at Low Temperatures." Prog. Org. Coat., 24 (1-4) 189-208 (1994)
(16.) European Union, "Commission Regulation (EU) No 1282/ 2011 of 28 November 2011 Amending and Correcting Commission Regulation (EU) No 10/2011 on Plastic Materials and Articles Intended to Come into Contact with Food." Off. J. Eur. Union, L328 22-29 (2011)
(17.) Hosseinpour, D, Guthrie, JT, Berg, JC, Stolarski, VL, "The Interfacial Interaction Contribution to the Mechanical Properties of Automotive Topcoats." Prog. Org. Coat., 54 (3) 182-187 (2005)
(18.) Hosseinpour, D, Guthrie, JT, Berg, JC, Stolarski, VL, "Effect of the Pigment-Polymer Interactions on the Mechanical Performance of Automotive Topcoat Layers." J. Appl. Polym. Sci., 101 268-2261 (2006)
S. M. Magami ([mail]), J. T. Guthrie
Department of Colour Science, School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
J. T. Guthrie
P. K. T. Oldring
The Valspar Corporation, Witney. Oxon 0X8 6XR. UK
The Food and Environment Research Agency, Sand Hutton, York Y041 1LZ. UK
Table 1: Properties of the Ti[O.sub.2] pigments Pigment Surface Specific Oil Grade treatment gravity absorption (g/[cm.sup.3]) (g/100 g) Ti[O.sub.2] 1 Alumina 4.1 19 Food grade Ti[O.sub.2] 2 Alumina 4.1 21 Food grade Ti[O.sub.2] 3 Alumina 4.0 19 Food grade Table 2: Properties of the amino crosslinkers, hexamethoxymethyl melamine (HMMM), hexabutoxymethyl melamine (HBMM), methylol melamine type crosslinker (MMTC), and dibutoxymethyl benzoguanamine (DBMB) Crosslinker Molecular weight (g/mol) Residual melamine (%) HMMM-1 390 0.12 HMMM-2 390 0.07 HMMM-3 390 0.18 HBMM-1 642 <0.1 HBMM-2 642 0.19 HBMM-3 642 <0.1 MMTC-1 471 0.15 MMTC-2 530 0.15 MMTC-3 530 0.14 MMTC-4 355 0.15 DBMB 360 0.18 Crosslinker Viscosity (Pa s) Key functional group(s) HMMM-1 5.07 -OMe HMMM-2 9.28 -OMe HMMM-3 5.52 -OMe HBMM-1 4.17 -OBu HBMM-2 2.65 -OBu HBMM-3 4.32 -OBu MMTC-1 3.50 -C[H.sub.2]OH, -OBu MMTC-2 1.56 -C[H.sub.2]OH, -OBu MMTC-3 25.37 -C[H.sub.2]OH, -OBu MMTC-4 3.65 -C[H.sub.2]OH, -OBu DBMB 1.22 -OBu, -NH Table 3: Titanium and aluminum content of the Ti[O.sub.2] pigments Ti[O.sub.2] pigment % Ti % Al Ti[O.sub.2] 1 50.10 1.58 Ti[O.sub.2] 2 47.46 1.40 Ti[O.sub.2] 3 52.13 2.18 Table 4: Effect of aging of the fluid epoxy-anhydride coatings on subsequent melamine release from the cured coatings and on the extent of crosslinker hydrolysis to yield melamine, from HBMM-1 crosslinked epoxy-anhydride coatings Age of liquid Ti[O.sub.2] 1 Ti[O.sub.2] 2 Ti[O.sub.2] 3 coating used used used Melamine released ([micro]g/6 [dm.sup.2]) 2 weeks 194 [+ or -] 2 195 [+ or -] 4 192 [+ or -] 1 20 weeks 332 [+ or -] 4 330 [+ or -] 2 328 [+ or -] 2 40 weeks 373 [+ or -] 3 371 [+ or -] 2 350 [+ or -] 3 % Hydrolysis of crosslinker to give released melamine 2 weeks 27 [+ or -] 1 28 [+ or -] 2 27 [+ or -] 2 20 weeks 47 [+ or -] 2 46 [+ or -] 2 46 [+ or -] 2 40 weeks 52 [+ or -] 3 52 [+ or -] 2 50 [+ or -] 1 Table 5: Effect of the presence of the Ti[O.sub.2] on the hydrolysis of epoxy-anhydride coatings to melamine Crosslinker in Melamine release formulation ([micro]g/6 [dm.sup.2]) With Ti[O.sub.2] Without Ti[O.sub.2] HMMM-1 404 [+ or -] 6 622 [+ or -] 4 HMMM-2 392 [+ or -] 3 525 [+ or -] 4 HMMM-3 418 [+ or -] 2 567 [+ or -] 5 HBMM-1 217 [+ or -] 2 321 [+ or -] 7 HBMM-2 423 [+ or -] 5 655 [+ or -] 6 HBMM-3 249 [+ or -] 4 373 [+ or -] 6 MMTC-1 227 [+ or -] 5 497 [+ or -] 6 MMTC-2 242 [+ or -] 2 294 [+ or -] 8 MMTC-3 224 [+ or -] 3 317 [+ or -] 7 MMTC-4 193 [+ or -] 4 371 [+ or -] 3 DBMB 79 [+ or -] 2 80 [+ or -] 8 Crosslinker in % Hydrolysis of crosslinker to formulation release melamine With Ti[O.sub.2] Without Ti[O.sub.2] HMMM-1 35 [+ or -] 1 74 [+ or -] 1 HMMM-2 34 [+ or -] 2 70 [+ or -] 2 HMMM-3 36 [+ or -] 2 75 [+ or -] 1 HBMM-1 31 [+ or -] 2 71 [+ or -] 1 HBMM-2 54 [+ or -] 4 96 [+ or -] 5 HBMM-3 36 [+ or -] 2 81 [+ or -] 1 MMTC-1 23 [+ or -] 6 78 [+ or -] 3 MMTC-2 28 [+ or -] 2 53 [+ or -] 2 MMTC-3 41 [+ or -] 2 37 [+ or -] 2 MMTC-4 15 [+ or -] 2 34 [+ or -] 2 DBMB 6 [+ or -] 2 10 [+ or -] 3
Please note: Some tables or figures were omitted from this article.
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|Title Annotation:||BRIEF COMMUNICATION|
|Author:||Magami, Saminu Musa; Oldring, Peter K.T.; Castle, Laurence; Guthrie, James Thomas|
|Publication:||Journal of Coatings Technology and Research|
|Date:||Nov 1, 2014|
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